Worst that I own? Probably the EF-S 18-55mm IS, followed by my 50mm f/1.8 II but these lenses are mostly bad in terms of build and handling. I'd love USM and FTM, then I might not mind them as much, but the focus motors are so bloody loud in certain situations it's almost comical.

Canon 18-5mm. I bought it with my DSLR but quickly upgraded to the 24-105mm. It's not great optically, the aperture is not very wide and the build quality is poor. Still it only cost me an extra £10 to get the DSLR with 18-55mm compared to the body only... Can't really complain!

This is definitly interesting, since it makes a change from the usual "what is the best ... ?" questions.

The worst Canon lens I have is probably the EF 28-80 V USM. It's basically a kit zoom for the EOS 300 low end film body. Used on a 5D (mk 1 or 2) it is never sharp by modern standards, but about what you'd expect from a film compact.

If we open it wider than that, then I'd have to guess older versions of that zoom, or other early EF kit zooms might be worse.

I *guess* it's the EF 35-80mm 4-5.6. By accident I got this lens twice. One of them has a broken part on the outside and the other one has a terribly scratched front element. I didn't compare it with the other lenses, yet. Then... well... hard to say. Didn't really compare 18-55mm 3.5-5.6 IS vs. 28-80mm 3.5-5.6 USM (first version with metal mount and ring USM).

I guess those are the worst three lenses of my collection, probably the 35-80mm as the far worst of them.

I think one of those kitzooms from the SLR age are some of the worst, referring to those 35-80 lenses etc.Of course while we're talking about the worst lenses, the 18-55 II do I not really consider as that ''bad''. When you know how to use it (not going past 35mm) and f/8 all the time (yes, that's a verb ) you can get some nice results. The 75-300 III is one Ive heard a lot of too! Seen some pictures after considering it when I wasnt that ''rich'' as Im right now...

Worst optics: The 75-300mm f4-5.6 III closely followed by the 28-80mm f3.5-5.6 II were the worst I've owned. They came with my old film Rebel 2000, which was my first SLR back in 2001. At the time, I thought they were the bee's knees, especially on 4x6" prints. 2010 I got a Rebel XS that came with an 18-55mm f3.5-5.6 IS and picked up a 50mm f1.8 II. I compared near-identical framed shots between the 4 lenses and saw how truly god-awful those 2 old lenses were in 10MP glory. I never attached them to a camera again.

Worst build quality: Oddly enough, the 50mm f1.8 II closely followed by the 18-55 f3.5-5.6 IS. It seems the optical quality was inversely proportional to the build quility on cheap lenses.

I see many of you curse the 18-55 IS. Mine is actually super sharp at f/7, between 20-50mm Razor sharp...Worst lens is 55-250 IS, not in therm of IQ, it was acceptable, but i always shot with that lens at 250mm and it was a nightmare to achieve focus very very slow...comparable to my 100mm f.2.8 macro though... at least it has superb IQ.Worst third party lens- Samyang 14mm f 2.8, i would get better images if I used a plastic bottle instead of that lens.

I don't think it's a bad lens and I quite like it. But out of my lenses it's not at the top end. My best two are the 100mm 2.8L Macro IS USM and the 100-400mm. The 12-24mm isn't really sharp but it's a lot of fun to use and the build quality is nice, the 24-105mm isn't that good in this case (I guess it has some kind of front focus but my 500D has no AF adjustment), the 50mm 1.8 has some great optics inside...

And the other EF(-s) lenses I have are the 35-80mm, the 28-80mm USM, the 18-55mm IS and the 100-300mm USM. The last one is pretty good, the 28-80mm has a very similar build quality but I didn't really compare the image quality, the 18-55mm IS is quite OK and the 35-80mm is absolutely no fun.

Erm... oh... I just remembered something... I also have the 28-105mm - some kind of lens whose build quality is very similar to the 18-55mm (non IS, first version) lens. Well - I could do a comparison between the 24-105mm 4L IS USM lens and this cheap 28-105mm 4-5.6 lens My brother has the 35-105mm 4.5-5.6 lens, which is probably the loudest lens I know.

With all my cheap lenses I could do some kind of "cheapest DSLR start with used equipment" test (cheap EF lenses vs. M42 lenses with adapters) and after that I could say which of them is the worst

From what I read, the 28-105mm has USM and is actually a decent lens, especially for the price it was released at.

Also, keep in mind that while the 18-55mm kit lens is not necessarily a bad lens, especially for the price, it can still be the worse lens that someone owns as they have spent a lot more money on all their other gear. They started with this lens and then upgraded from it. No one I know would willingly spend money to downgrade the quality of their pictures unless they were going for a specific effect.

Now, what I would think would be a slightly more interesting question would be what is the worse lens in your bag that you actually use? Having a lens tucked away in some corner that you would probably never use again is easy to label as being the worst. Actually chosing between all your lenses that you love to determine the worst is harder to do.